Probleme Null Pointer Exception

Bonjour tout le monde !

J'ai cette erreur qui s'affiche quand je fais une requete pour select from d ma base de données.

voilà l code :

      public List chercheravance(String critere) throws java.lang.ClassNotFoundException,SQLException,DonneesIncompletesException{
 
          int i=0;
                 List v = new Vector();
          try {  
 
                  //ResultSet r=s.executeQuery("Select * from materiel where " + critere );
                  Query="Select * from materiel where "+critere;
                  System.out.println("requete:"+Query);
                  r=s.executeQuery(Query);
                   // getS().executeUpdate(Query);
 
                  while (r.next()) {
                     materiel e=new materiel();
 
                     e.setModel(r.getString("model"));
                     e.setTypeM(r.getString("type"));
                     e.setSerial(r.getString("serial"));
 
                     System.out.println("m"+r.getString(model));
 
 
                     v.add(i,e);
                     i++;
            }
 
        } catch (SQLException ex) {
          System.out.println("fonction chercher : "+ ex.getMessage());
        }
     return v;
     }

et voilà ma servlet doPost

protected void doPost(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
 
 
 
            String Serial=request.getParameter("serial");
            String critere="";
 
        if (Serial != null && !Serial.equals("")){ critere =  "serial='" + Serial + "'and "; }
        critere = critere + "1=1";
 
 
            try {
                    recher e = new recher();
 
 
                    List v = e.chercheravance(critere);
 
                HttpSession session=request.getSession(true);
                session.setAttribute("bean4",v);
                getServletContext().getRequestDispatcher("/RechercheSR.jsp").forward(request,response);
 
            } catch (ClassNotFoundException cl) {
            Logger.getLogger(RechercheS.class.getName()).log(Level.SEVERE, null, cl);
        }catch(recher.DonneesIncompletesException d) {
            Logger.getLogger(RechercheS.class.getName()).log(Level.SEVERE, null, d);
        }catch (SQLException sql) {
            Logger.getLogger(RechercheS.class.getName()).log(Level.SEVERE, null, sql);
        }catch(NullPointerException n){
            Logger.getLogger(RechercheS.class.getName()).log(Level.SEVERE, null, n);
        }
 
 
 
          }

et vOILç l erreur q s affiche

GRAVE: null
java.lang.NullPointerException
at metier.recher.chercheravance(recher.java:112)
at gpi.RechercheS.doPost(RechercheS.java:55)
at java.lang.Thread.run(Thread.java:619)